The goal of the proposed research is to elucidate the mechanism whereby vasopressin promotes transcellular water movement in responsive epithelial tissues. The specific objective is to determine the role of microtubules and microfilaments in this action of vasopressin, and to examine the possibility that the cytosolic calcium ion concentration plays a critical role in the action of the hormone. A combined functional, biochemical and morphological approach will continue to be employed in studies on the toad bladder, in an attempt to determine (a) how microtubules participate in the action of vasopressin, (b) whether - and, if so, how - microfilaments are involved, (c) whether compartmentalization of ionized calcium plays a role in the action of the hormone (in turn regulating microtubule assembly and/or microfilament function?). Functional studies of the movement of water, sodium, and urea in response to vasopressin will be carried out in isolated toad bladders; the effects of agents which influence (1) microtubule structure and/or function, (2) mitochondrial Ca ions uptake, will be examined. Biochemical studies of oxygen consumption and calcium uptake by mitochondria isolated from the bladder epithelial cells will be performed. Electron microscopic studies of the state of assembly and distribution of microtubules and of actin filaments, in the bladder epithelial cells will be carried out, using stereological and immunocytochemical techniques.